The present disclosure relates to a lens with a first lens unit, at least one second lens unit and a pupil. In addition, the present disclosure relates to an optical observation device.
Optical observation devices typically include lenses optimized for the particular purposes of use. Optical observation devices can include, for example, microscopes, endoscopes, etc. Adding another optical element into the lens requires a new design of the lens. In other words, each lens cannot be flexibly adapted to other observation conditions. As a result, each lens can only be optimally used in a narrow field of observation conditions.
For example, microscopy includes high-aperture lenses such as, for example, immersion lenses. In immersion lenses, a liquid film is present between the lens and the cover glass covering the specimen is associated with a high lateral resolution with a low depth of field on the object side. While the smallest lateral, resolvable structure dmin varies in diffraction-limited resolution according to the equation
      d    min    ∼      λ    NA  
at a wavelength λ proportional to the inverse value of the numeric aperture NA, the longitudinal resolution I, also called the depth of field, varies according to the equation
  l  ∼      λ          NA      2      
proportionally to the inverse value of the square of the numeric aperture. The depth of field given by the last equation for a wavelength λ is also called the Rayleigh length or Rayleigh unit (RE). As can be seen from the two equations, the usable depth of field rapidly decreases when the numeric aperture (NA) is raised to improve the lateral resolution. In order to be able to detect three-dimensionally expanded specimen areas, for example, entire cells or cellular organelles in natural tissue embedded in a physiological solution of common salt, and/or image information of the complete object volume with diffraction-limited resolution in microscopy, it is therefore necessary to record up to several hundred so-called Z sections. A Z section represents a recording with high lateral resolution in a given focus position, i.e., in a certain depth position of the object. A two-dimensional image with elevated depth of field that can be represented on an image display device can be produced from the individual Z sections, if necessary, with image processing methods. The focus positions of the Z scan can be adjusted by refocusing the microscope.
However, a refocusing of the microscope, for example, adjusting the object stand relative to the microscope, can induce large image errors in the microscope lens since the lens is optimized for a certain focus position. The changing of the lens in such a manner that it is optimized for another focus position requires as a rule an extensive changing of its optical components.
The image errors produced during the refocusing can be subdivided conceptually into image errors that have amounts that are large but can be calculated in advance at an average wavelength and into image errors that also have a chromatic dependency that can also be calculated in advance. Examples for the first type of image errors are different orders of the opening error, that is, different orders of the spherical aberration. Examples for the second type of image errors include the chromatic longitudinal deviation CHL as the primary, secondary and tertiary spectrum. In a similar manner image errors are induced if the refocusing takes place where the intercept length of the lens is altered with the aid of an internal focusing lens. In both instances the image quality is deteriorated drastically upon a slight change in the object distance or of the intercept length. A slight change occurs upon a changing of the object distance or a changing of the intercept length of a few Rayleigh units. However, the adding of elements, which might compensate the deterioration of the image quality, requires an extensive change to the optical design of the lens.